With magnetic bearings contactless suspension can be obtained. Their limited friction losses make them attractive for high-speed applications. The design of rotary high-speed machines is often complicated due to rotor dynamic limitations. In that sense, any reduction of the axial length of a shaft contributes to the rotor dynamic margin. This property is maximally exploited in so-called combo bearings. These are bearings wherein the design integrates axial and radial channels in a compact arrangement in which several functional parts are shared.
Various examples of combo bearings can be found in patents and literature. Often, the path of the axial control flux crosses the central hole of a laminated stack of ferromagnetic material. Examples of this can be found in patents or patent applications U.S. Pat. No. 5,514,924, U.S. Pat. No. 6,268,674, U.S. Pat. No. 6,727,617, WO 2008074045, CN 1737388. Other examples are found in literature, as e.g in the papers by Imoberdorf et al., Pichot et al. and Buckner et al. In combo bearings of the type depicted in patent U.S. Pat. No. 6,359,357 B1 of Blumenstock, the axial control flux does not cross the central hole of a laminated stack of ferromagnetic material.
The axial channel performance of a combo bearing may be adversely affected if the path of the axial control flux crosses the central hole of a laminated stack or, more generally, if a combo bearing contains an area where an electrically conductive path surrounds the control flux. In that case, varying control fluxes can induce voltages in the surrounding material. These induced voltages cause circulating currents, thus Joule losses as well, if the surrounding path is closed and electrically conductive. Actually, such a laminated stack can be considered being a short circuited secondary coil of a transformer, the axial control coil being the primary coil. The effect is frequency dependent: the loss grows with frequency. Given a particular axial control current and frequency, Joule losses reduce the force that can be realized. Consequently, the performance of the axial channel is affected.
Similar phenomena may occur in the lamination stack on which the axial actuator acts. In that case the control flux enters the stack itself, but the physical explanation is the same. In patent U.S. Pat. No. 6,268,674, Takahashi proposes to cut a series of evenly distributed radial slits inside such a target lamination stack. Obviously, in order to maintain sufficient strength while rotating, the laminations are not cut over their entire thickness. By doing so, induced currents remain local, provided the control flux exclusively enters the slit region. This technique only provides a solution for reducing the losses in the target lamination stack. The global control flux is still surrounded by the stator stack.
To our knowledge, other techniques for reducing this kind of losses have not been reported. In this patent, a different technique for loss reduction is presented. It may be applied to both rotor and stator stacks of a combo magnetic bearing.